Field of the Disclosure
Embodiments of the present invention relate to an organic light emitting display device which facilitates to improve picture quality by improving an aperture ratio of a pixel, and to reduce a manufacturing cost by decreasing the number of channels in a data driver.
Discussion of the Related Art
With the advancement of an information-oriented society, various requirements for the display field to visually express an electrical information signal are increasing rapidly, and thus, research is being conducted on various flat display devices that are thin, light, and have low power consumption.
For example, the flat display devices may be liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, organic light emitting display (OLED) devices, etc. Among these flat display devices, the organic light emitting display (OLED) device has been attracted as a next-generation flat panel display owing to advantages of rapid response speed and low power consumption. In addition, the organic light emitting display device can self emit light.
FIG. 1 illustrates a pixel circuit and an organic light emitting diode provided in a pixel of an organic light emitting display device according to a related art. FIG. 1 illustrates an equivalent circuit of one pixel among a plurality of pixels provided in a display panel.
Referring to FIG. 1, the organic light emitting diode (OLED) provided in each pixel of the organic light emitting display device is electrically connected between a cathode power source (EVSS) and a source terminal of a driving thin film transistor (driving TFT, DT), whereby the organic light emitting diode emits light by a data current (I_oled) supplied from the driving TFT (DT). The organic light emitting diode (OLED) emits light by controlling a level of the data current (I_oled) flowing from a first driving power (EVDD) terminal to the organic light emitting diode (OLED) through the driving TFT (DT), to thereby display a predetermined image.
Due to non-uniformity in a manufacturing process of a thin film transistor (TFT), the properties of threshold voltage (Vth) and mobility of the driving TFT (DT) and first and second switching TFTs (ST1, ST2) included in the pixel circuit may be differently shown by each pixel. Thus, even though a data voltage (Vdata) is identically applied to the driving TFT (DT) for each pixel, a deviation of current flowing in the organic light emitting diode (OLED) occurs so that it is difficult to realize uniformity in picture quality.
Also, the driving TFT may have a problem relating to a deterioration of the properties of a threshold voltage and a mobility with the lapse of driving time. This deterioration becomes more serious with a lapse of driving time. Thus, even though the data voltage (Vdata) is applied identically, the current flowing in the organic light emitting diode (OLED) is gradually reduced so that a luminance becomes low.
In order to overcome these problems, there has been developed a method for sensing the properties of threshold voltage and mobility in the driving TFT (DT) of each pixel, and compensating for the change of properties in the driving TFT (DT) by an external compensation method.
A sense signal line (SL) is formed in the same direction as that of a gate line (GL), and a second switching TFT (ST2) is formed and switched by a sense signal (sense) applied to the sense signal line (SL). As a pre-charging voltage is supplied to a pixel to be sensed, and the second switching TFT (ST2) is selectively switched, the data current (I_oled) supplied to the organic light emitting diode (OLED) is supplied to an analog-to-digital converter (ADC) of a drive IC via a reference power line (RL).
After sensing the threshold voltage and mobility of the driving TFT (DT), the data current is converted into compensation data corresponding to the change of threshold voltage and mobility of the driving TFT of the pixel (P) through the use of the analog-to-digital converter (ADC) based on the sensing result value.
In the organic light emitting display device according to the related art, the reference power line (RL) is formed to sense the change of properties in the driving TFT (DT) of each pixel, whereby an aperture ratio of the pixel is lowered.
As a reference voltage (Vref) is supplied to the reference power line (RL), the number of channels in a data driver (D-IC) is increased so that a manufacturing cost of the data driver (D-IC) is increased, thereby increasing a manufacturing cost of the organic light emitting display device.
FIG. 2 is a waveform diagram of the sense signal in the organic light emitting display device according to the related art.
Referring to FIG. 2, a real-time sensing process is carried out by supplying the sense signal to each sense signal line or several sense signal lines by each line unit or each group unit.
In order to reduce the sensing time, when the sense signal is supplied to the sense signal line, the consecutive two sense signals are overlapped by ½ time. That is, the (N−1)th sense signal and the N-th sense signal are overlapped by ½ time, and the N-th sense signal and the (N+1)th sense signal are overlapped by ½ time, to thereby sense the properties (threshold voltage and mobility) of the driving TFT (DT) for the entire pixels.
For the overlap driving of the sense signal, when a short occurs between the reference voltage (Vref) and low-potential driving voltage (EVSS) in the N-th line, the reference voltage (Vref) is lowered to the low-potential driving voltage (EVSS) (Vref→EVSS), and Vgs voltage of the (N−1)th line becomes high, whereby a luminance of each organic light emitting diode (OLED) in the pixels formed in the (N−1)th line becomes high. Accordingly, a luminance deviation occurs between the pixels of the prior sense signal line and the pixels of the next sense signal line with respect to the corresponding sense signal line, thereby causing deterioration of picture quality, for example, spots on a screen.